1. Field of the Invention
The invention generally relates to procedures aimed at maintaining the reliability of data transmission in wireless telecommunications systems comprising several base stations, and in particular to maintaining the composition of the data carried in data cells during a handover in a system which transfers data as data cells whose relative order is significant.
2. Prior Art
The ATM (Asynchronous Transfer Mode) is a strong candidate for a fast future transfer protocol in B-ISDN networks (Broadband Integrated Services Digital Net-work), for example, and generally in communications between data transmission devices. The network consists of nodes and terminals, and of links between them. In an ATM network, data is transferred as cells in digital form, each cell comprising a so-called payload of 48 bytes and a header of 5 bytes. In order for the amount of the header information to be kept at a minimum, the headers do not contain complete routing information between the transmitting and the receiving devices but only information on the virtual path and channel in which the respective data transfer connection is carried. The nodes of the network contain the necessary routing information on the basis of which the respective identifiers of the virtual path and channel are interpreted as a reference for the next respective node. The relative order of the cells must remain the same throughout the transmission because there is no specific mechanism in the ATM network that can be used to rearrange out-of-order cells.
Traditionally, ATM-links have been conceived as wired circuits or optical cable connections, whereby the above-mentioned requirement of the relative order of the cells has not caused major problems. However, it is anticipated that the terminals of future data transfer solutions must have the same kind of mobility and independence of permanent connections as mobile phones of cellular networks already have. Because of its effectiveness and the provision to flexibly modify it, the cellular radio network is a likely architecture in the future. A typical cellular network comprises several base stations (BS) which are affiliated with--possibly through a base station controller (BSC)--a mobile switching center (MSC). In a large cellular network there are numerous mobile switching centers as well as base stations and base station controllers that work under the mobile switching centers. The transmission range of each base station, i.e., the geographical area where the terminals can communicate wirelessly with the said base station, is called a cell.
However, to avoid confusion, the term cell is in this patent application used to refer only to the transferred data unit defined by a given data transmission system, the length of the unit being the above-mentioned 53 bytes in the ATM system which is used as an example.
A typical phenomenon of cellular systems is the change of base stations, i.e., the handover, where a given mobile terminal moves from the transmission range of a first base station to that of a second base station, whereby essentially all the data transmission between the terminal in question and the network is rerouted through the new base station. A perfect synchronization of the operation during the handover is often not possible, which may result in packet loss, duplication or misordering. This is especially severe in the ATM system where a successful transmission of all the cells and ordering are a prerequisite for effective data transmission.
A handover may take place without a notice or with a notice. The former refers to a situation where the connection between the base station and the terminal breaks quickly and the latter implies that, where the connection worsens gradually, one can prepare for procedures required for handover. The doctoral thesis of S. K. Biswas "Handling Realtime Traffic in Mobile Networks" (University of Cambridge, September 1994) discloses procedures for implementing a handover both without a notice and with a notice. In order to explain the background of the invention, the handover of base stations is described briefly in the following with reference to FIGS. 1 and 2.
FIG. 1 presents the mobile unit MU, two base stations BSp and BSq, and mobile representative MR which is in charge of routing the traffic between the said base stations and the rest of the data transmission network. In addition, the data transfer segments between the said devices are designated letters i, j, k, and l. The data transfer segment directed from mobile representative MR to the rest of the network is designated s. The data directed from mobile unit MU to the network is called uplink data and the data propagating in the opposite direction is called downlink data.
FIG. 2 presents signals between the said devices by means of numbered arrows, relating to the handover with a notice in which BSp is the old base station and BSq is the new base station. Mobile unit MU stops transmitting uplink cells along segment i and transmits initiate message 21 to new base station BSq BSq transmits forward message 22 to mobile representative MR on the need to reroute. As a response, mobile representative MR stops sending downlink data, which arrives along segment s, to old base station BSp along segment k, and directs it to new base station BSq through segment l. Before transmitting the actual data to be transmitted, mobile representative MR transmits reply message 23 which is forwarded to the mobile unit as ack message 24 by base station BSq. At the same time, the new base station creates a temporary FIFO buffer (first in, first out) 25 for storing the downlink data that comes from mobile representative MR. It is not until mobile unit MU believes it has received all the downlink data which had been directed from mobile representative MR via segment k to old base station BSp before switching the connection to segment l, that it transmits done message 26 to new base station BSq and starts to transmit uplink data. As a response to done 26, new base station BSq starts sending downlink data (by first unloading buffer 25) and, for its part, finishes the handover by sending complete message 27 to mobile representative MR.
The connection from mobile representative MR to old base station BSp can be terminated by using a ready message immediately after the data transfer connection of segment k has been broken off (arrow 28), or once ready message 27 has arrived (arrow 29). For the sake of clarity, FIG. 2 presents the interrupt time of the uplink data, represented by a shaded bar, the interrupt time of the downlink data in mobile representative MR, represented by a black bar, and the interrupt time of the downlink data in new base station BSq, represented by a white bar.
According to Biswas, the handover without a notice comprises several similar procedures, but disruptions in data transmission can be longer because delay is caused in the time spent before a break is detected in the connection passing through the old base station. In both the arrangements, the one with a notice and the one without a notice, it is possible that there is packet loss or packet misordering. In the case of uplink data, it is possible that mobile representative MR breaks off the data transmission from segment k to the network before all the uplink cells have been transmitted across segments i and k. After the connection is terminated, the cells in question are considered lost, or if they are still directed to the network, they can be out of order in the uplink. The downlink cells, which have been directed to segments k and i before the handover, may be lost altogether because of a weak radio link, or they can arrive late in mobile unit MU, causing out-of-ordering in the downlink.
Packet loss and out-of-ordering usually results in that an erroneous checksum or other indicator is detected on a higher protocol level, whereby a given protocol data unit (PDA) comprising several cells is denied and requested to be sent again. This is uneconomic from the point of view of the network resource utilization.
Patent publication EP 426 269 to British Telecommunications discloses a method in which the base stations are assembled in groups of several base stations. To facilitate and speed up the handover, all the cells directed to a mobile unit in the transmission range of a given base station are sent to all the base stations of the same group by means of the data transmission network. The publication presents an idea according to which the old base station sends all the cells delivered to it, and after this, the new base station starts sending the cells delivered through it. The publication also presents a predictor which is incorporated in the base station controller or in the mobile representative that controls the base stations. The task of the predictor is to monitor the movement of the mobile unit from one transmission range to another and to estimate as to where the mobile unit will migrate next. This is effected in order to decrease the amount of cells sent unnecessarily to the remotest base stations of the group. However, the solution cannot maintain packet ordering or prevent packet loss because the old and the new base stations cannot tell precisely which cells have been transmitted and received correctly immediately before or after the handover.
Patent publication EP 366 342 to AT&T discloses a method in which data is transmitted as cells in a cellular radio network and the header segment of each cell comprises an unchanging segment which remains unmodified regardless of the changes in routing, and a changing segment whose contents vary during a handover or during other rerouting processes. The publication suggests that the arrangement can be used to facilitate the handover, but only the definition of the routing is benefited. A similar method is disclosed in patent publication EP 577 959 to Roke Manor Research Ltd., relating to the ATM network in particular. In this case, the unchanging segment in the cell header is a so-called VCI field (Virtual Channel Identifier) and the changing segment is a so-called VPI field (Virtual Path Identifier). Neither one of the publications discloses a method which could ensure that packet ordering remains the same and/or which could prevent packet loss during the handover.
Patent publication EP 577 960 to Roke Manor Research Ltd. discloses a method in which at least one of the base stations of a cellular network is affiliated, through the ATM network, with at least two mobile representatives which, in the disclosed arrangement, also function as repeaters between the ATM network and a fixed telephone network. The idea is to arrange the VPIs and the VCIs of the ATM network which control the routing so that even though a given mobile unit moves into the transmission range of another mobile representative (or mobile services switching center), the routing is implemented by linking via the original mobile representative. The arrangement has some advantages in decreasing the amount of switchings in the network but it does not have an effect on packet loss between the base stations and the mobile unit and/or on the delay of cells during a handover.
The article "BAHAMA: A Broadband Ad-Hoc Wireless ATM Local-Area Net-work", Proc. ICC '95, Jun. 18-22, 1995, Seattle, written by K. Y. Eng et al., discloses a method in which the GFC field in the header segments of ATM cells is used to implement cell-oriented sequential numbering. The purpose is to contribute to the synchronization and combination of steams of cells that arrive at a given conjuction point along two parallel routes. The purpose in numbering the cells is aimed particularly at identifying them unequivocally, so that cells are not duplicated or lost when the streams of cells are combined and their order will remain the same. In this case, a problem might occur because only numbers from 0 to 15 can be presented in the GFC field which has a maximum of four bits, whereby the numbering cycle remains so short that cells with the same number belonging to subsequent cycles may become out-of-ordered.